The use of multiple antennas at both the transmitter and receiver of a wireless communication system along with related baseband signal processing is referred to as Multiple-Input Multiple-Output (MIMO) technology. In a MU-MIMO system, precoding is applied at the transmitter in order to suppress mutual interference experienced by each receiving station caused by transmissions to other receiving stations. To those skilled in the art, MU-MIMO precoding, refers to spatial encoding of the transmitted signal based on propagation channel. In order to apply MU-MIMO precoding, the transmitting station is required to know the Channel State Information (CSI) of the radio channels connecting it to each of the receiving stations for transmission. In 3GPP LTE systems, it is common for the receiving stations (e.g., user equipments) to measure CSI and report CSI to the transmitting station (e.g., eNodeB) via an uplink feedback channel. The content of CSI feedback contains RI (rank indicator), CQI (channel quality indicator), and PMI (precoding matrix indicator) for each downlink component carrier (CC).
In 3GPP LTE systems, CSI reporting is categorized into two types. A first type is called Aperiodic CSI reporting, which applies a triggering mechanism. When triggered, CSI is either reported for all activated downlink CCs, or reported for the downlink CC that is linked to the uplink CC for which the grant containing the trigger is sent. Aperiodic CSI is carried on PUSCH only. A second type is called Periodic CSI reporting, which applies an upper layer (e.g., RRC layer) configuration mechanism. When configured, CSI is reported periodically for one activated downlink CC only. Periodic CSI is carried on PUCCH or PUSCH.
In Rel-8/9 LTE systems, periodic CSI reporting for RI and CQI/PMI is configured separately for a single component carrier. RI reporting is configured by the periodicity MRI and relative offset MOFFSET, RI. CQI/PMI reporting is configured by the periodicity NP and relative offset NOFFSET, CQI. When the relative offset values for RI and CQI/PMI reporting are not set perfectly, collision of RI and CQI/PMI may occur for single-carrier cases. In general, the priority rule is that RI is prioritized over CQI/PMI in case of collision. Therefore, when both RI reporting and CQI/PMI reporting are configured at the same time slot, RI reporting is sent and CQI/PMI reporting is dropped by the UE.
In Rel-10 LTE-A systems, carrier aggregation (CA) can support up to five DL CCs so CSI reporting for multiple DL CCs is possible. Periodic CSI reporting, however, is configured for one activated DL CC only, which is reported via the primary uplink CC. There are 10 feedback types and 4 feedback modes defined in Rel-10 LTE-A. It is thus desirable to have a detailed priority rule to decide for which DL CC the configured periodic CSI reporting will be transmitted when there is periodic CSI reporting collision across the multiple DL CCs.